An early circa 1960 installation by Pasveer for waste water, i.e. primarily sewage, purification by the activated biological sludge method included a closed circuit or ditch with a horizontally-rotated brush rotor used for adding needed oxygen (air) to the sewage and moving the sewage in circulation. In U.S. Pat. No. 3,510,110, an orbital system employing an elongated tank with central partition was disclosed which employed a vertically-rotated surface aerator located at the end(s) of the partition wall for both aerating the sewage and circulating the sewage around the channels formed by the patition wall and the sides of the tank. This latter system has had great commercial success with over 500 plants in operation world-wide (ranging from less than 1 MGD capacity to one of over 10 million population equivalent) with over 100 plants in operation or in various construction phases since 1976 in the United States up to the present time. Sold under the trademark "Carrousel.RTM.", the high popularity of the system is due primarily to its cost-effectiveness, simplicity of design, ease of operation and maintenance, and excellent effluent quality. It can treat raw domestic water to EPA advanced secondary standards without primary clarifiers or effluent filters. With extended aeration, it produces a highly stable water sludge requiring little or no further processing prior to disposal. Carrousel systems can be designed to have a power turn-down of 50-75%. Aerator drive horsepower can be varied from 100% of installed capacity to as little as 25% without loss of mixing and continuing sufficient mixed liquor channel velocity. This power down flexibility provides an ability to closely match oxygen input to the mixed liquor to oxygen demand of the microbes acting to degrade the sewage, without loss of mixing and movement. In the largest U.S. installation, over 25 MGD of sewage is treated in four units having twenty aerators utilized to aerate and circulate sewage through twenty-four channels formed by twenty partitions and exterior encircling concrete walls forming four tanks.
In U.S. Pat. No. 3,900,394, an orbital system, including a surface aerator and partition wall, also utilizes a separate mixing and propulsion means or uses units alternately as propulsion-mixers and as aerators, as desired.
One of the limitations of the present systems has been the depth of the channels which must be somewhat shallow to allow the surface aerator(s) to impel the mixed liquor along the whole effective cross-sectional depth of the channel without a build-up of settled solids on the bottom. Such settling would lessen the available volume in the circuit for treatment purposes by taking up a proportion of the original volume, effectively wasting the additional costly concrete and land area utilized in making the originally sized channels. Further, as with all systems, sellers, operators and users are concerned with maximizing the efficiency of their equipment and minimizing operating costs at all times during the system operation.
Mechanical surface aerators have been utilized for some years for oxygen (air) transfer into large lagoons or nonorbital tanks either suspended from fixed bridges in a tank or on floating platforms anchored by piles or cables. By changing the ballast in float pontoons, the aerator submergence can be altered and the horsepower draw changed as the aerator rises and falls as the liquid level changes. Rotational speed or submergence of the aerators by a weir level adjustment also can be varied to change the impeller horsepower draw. The typical impeller for these applications includes an inverted cone impeller plate having a series of radial impeller blades extending downwardly from the underside of the conical impeller plate. These aerators are used for aerating and mixing of waste water in the tank or lagoon. Normally a grid of aerators are positioned over the lagoon surface area. If lagoons, basins or tanks are too deep to provide proper mixing, it has been contemplated to provide a four or six-bladed submerged mixing turbine attached to the bottom of the surface impeller at a position at the mid-depth of the basin. Draft tubes and baffles, to prevent excessive vortexing and provide smooth hydraulic flow to the aerator impeller, have also been employed in mechanical surface aerators in nonorbital systems.